122 BOTANY. [chap, i v. 



It has been already stated that on the evolution and 

 perfection of the sexual mode of reproduction, the more 

 antiquated asexual mode did not at once disappear, but 

 lingered on, and manifests itself even amongst the most 

 advanced of dry land plants, yet from a broad point of 

 view it is seen that the asexual mode of reproduction 

 degenerated in proportion as the sexual mode became 

 perfected ; and in many of the higher plants, where the 

 asexual mode is at the present day predominant, as in 

 many bulbous plants, this is due to a resuscitation of the 

 old method necessitated by surroundings being unfavour- 

 able for the continuance of the sexual method. 



Amongst Cryptogams we find as a rule the asexual 

 and sexual modes of reproduction both present, and in 

 many groups occurring at definite periods in the life- 

 cycle of the individual. This regular alternation of the 

 two modes is known as Alternation of Generations, a 

 phenomenon clearly illustrated in the ferns and mosses. 

 It is generally known that towards the autumn the under 

 surface of the fronds of ferns bear dark brown patches, 

 either as rounded spots or elongated streak-like lines, 

 depending on the kind of fern examined ; these patches 

 consist of exceedingly minute one-celled reproductive 

 bodies called spores, that are produced in numbers in 

 special structures called sporangia. Each spore, when 

 placed under favourable circumstances, is capable of 

 germination and producing eventually a new fern plant. 

 On germination the spore first gives origin to a slender 

 filament which, by cell-division at the tip, soon expands 

 and forms a small structure lying flat on the ground 

 and rarely exceeding a quarter of an inch in diameter ; 

 this is called a prothallus, and constitutes the first or 



